Microelectromechanical device for out-of-plane motion detection
The disclosure relates to a microelectromechanical device where the device structure includes a rotating mass structure and a linear mass structure. The rotating mass structure is formed of two rotating mass parts elastically coupled to the support through one or more springs that enable rotary motion of each of the rotating mass parts about respective rotary axes that extend parallel to each other along a first in-plane direction (IP1). The linear mass structure includes at least one elongate rigid body that extends in a second in-plane direction (IP2). One end of the linear mass structure is coupled to the first rotating mass part and the other end of the linear mass structure is coupled to the second rotating mass part such that rotary motions of the first and second masses result into linear motion of the linear mass structure in the out-of-plane direction (OP).
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The present disclosure relates to microelectromechanical devices, and more particularly to devices configured for detecting accelerations in out-of-plane direction.
BACKGROUND OF THE DISCLOSUREA microelectromechanical device can be made to move with an object to measure acceleration upon it. For this, the micromechanical device typically includes inertial mass structures, and acceleration forces acting on the micromechanical device can be measured by detecting displacements of these masses relative to fixed electrodes.
The microelectromechanical device comprises a support 100, and a device structure 102. The term support 100 refers herein to a rigid mechanical element that may be fixed to move with the measured object. Accordingly, the support is a structural element that provides a rigid, locally inert reference for movable elements of the microelectromechanical device. Movable elements of the microelectromechanical device can be suspended through deformable elements from the support. In a wafer structure, the support may be provided by a base layer underneath and separated by an out-of-plane gap from a structure layer into which the movable or deformable elements are patterned. The support may alternatively, or additionally, be provided by a capping above and separated by an out-of-plane gap from a structure layer into which the movable or deformable elements are patterned. The support and elements in the structure layer may, for example, be coupled to each other through one or more fixing points protruding from the support. Alternatively, or additionally, the support may include a frame that is rigidly fixed to the base layer and surrounds some or all elements in the structure layer.
The term device structure 102 refers herein to a combination of rigid and flexible elements that are jointly configured to undergo a defined mode of motion induced by acceleration acting upon the microelectromechanical device. In other words, the device structure is coupled to the support such that forces by accelerations in a measured direction induce the defined mode of motion. On the other hand, displacements or deformations of the device structure, caused by accelerations in any other directions, are minimized. In the defined mode of motion, the displacements of at least part of the device structure can be capacitively detected and transformed into an electrical signal that very accurately represents the measured acceleration.
In an initial static state of the device, a plane of the device structure 102 forms a reference plane 104 for the device. The initial static state refers here to a situation where the device structure is suspended from the support, is acted upon by gravitation, but is not subject to any induced accelerations. When the device structure is a patterned element, the reference plane may be considered to align with a surface of the planar layer from which the device structure is patterned. The surface considered as the reference plane is advantageously on the side that determines the distance that separates electrodes applied capacitive detection. In the example of
In capacitive measurements, acceleration is detected from change in the capacitance of an inertial mass that moves with respect to a static reference. In the microelectromechanical device of
However, structure layer thickness creates challenges in design of combined configurations where in-plane and out-of-plane detection is applied. A spring patterned into a thick structure layer is relatively loose to torsional motion but does nor deform easily in the out-of-plane direction. However, as may be understood from
As another aspect, a single seesaw mass in an out-of-plane sensing accelerometer component has to be sensitive to out-of-plane linear acceleration, but in practice it is always also somewhat sensitive to some in-plane linear acceleration, depending on the orientation of the torsional springs. Conventionally undesired sensitivities have been dealt with in design by ensuring that parasitic resonances are much higher in frequency than the resonance frequency of the measurement mode. In practice, the first parasitic resonances can be made up to 10 times higher in frequency than the resonance frequency of the measurement mode. However, for some applications, even this difference is not enough. For example, sensors of automotive products operate in very vibration-rich environments and with new applications, performance requirements are continuously tightening.
The lowest parasitic modes for torsional mode of motion sensitive to out-of-plane linear acceleration are in-plane and out-of-plane rotational modes. A parasitic in-plane rotational mode relates here to rotation of the mass of the rotor electrode 108 about an axis that is perpendicular to the reference plane (Z-axis). In a single uncoupled seesaw mass of a rotor electrode 108, the in-plane rotational parasitic mode is easily excited with linear in-plane acceleration excitations. This is because the mass of the rotor electrode is typically suspended by coaxial torsion springs, and the distance between the center of mass of the rotor electrode and the axis of the torsion springs acts as a lever arm for the in-plane rotational motion. Linear vibration is usually the most severe form of undesired excitation and it is typically present in many applications.
A parasitic out-of-plane rotational mode refers here to rotation of the mass of the rotor electrode 108 about an axis that is parallel to the reference plane (X or Y-axis). It is evidently difficult to design a multi-axis accelerometer in which a mass enabled for out-of-plane torsion mode would be sensitive only to linear out-of-plane acceleration. Sensitivity to other linear accelerations could be eliminated only by positioning the center of the mass of the rotor electrode on the axis of the torsion springs. However, by doing so, one would also loose sensitivity to the out-of-plane linear accelerations.
BRIEF DESCRIPTION OF THE DISCLOSUREAn object of the present disclosure is to provide device that overcomes or at least alleviates the above described problems in design of out-of-plane detecting device structures.
The object of the disclosure is achieved by the device structure characterized by what is stated in the independent claim. The preferred embodiments of the disclosure are disclosed in the dependent claims.
The disclosure is based on the idea including in the device structure a linear mass and two rotating masses, coupled to each other in a specific manner to apply benefits of rotary modes of motion and linear modes of motion.
In the following the disclosure will be described in greater detail by means of preferred embodiments with reference to the accompanying drawings, in which
In the example of
The second rotating mass part 212 has the same form as the first rotating mass part 210, but the U-shaped mass element is oriented in opposite direction, as shown in
In the disclosed arrangement the springs 214-1, 214-2 of the first spring structure couple the first rotating mass part 210 to the support and enable rotary motion of the first rotating mass part 210 about the first rotary axis 218. The springs 216-1, 216-2 of the second spring structure couple the second rotating mass part 212 to the support and enable rotary motion of the second rotating mass part 212 about the second rotary axis 220. The first rotary axis 218 and the second rotary axis 220 are in-plane axes, i.e. extend parallel to the reference plane. The first rotary axis 218 and the second rotary axis 220 are also parallel to each other, in this example they extend in the first in-plane direction IP1. In this disclosure, parallel orientation of axes means that they can be the same distance apart at every point along their whole length. The distance may also be zero, in which case the axes are aligned to one line. As mentioned earlier, the first spring structure 214-1, 214-2, and the second spring structure 216-1, 216-2 are functionally separate so that without further connecting parts, the first rotating mass part 210 and the second rotating mass part 212 could move independent of each other according to accelerations upon the device.
However, in the present device structure, motions of the first rotating mass part 210 and the second rotating mass part 212 are not separate but coupled in a specific way by the linear mass structure 208. The linear mass structure 208 includes at least one elongate rigid body that extends lengthwise in the second in-plane direction IP2.
The first end part 232 of the first rigid body 230 is coupled to the first section 222 of the first rotating mass part 210 through one or more springs 240-1. The springs 240-1 are configured to couple the first section 222 of the first rotating mass part 210 to move in the out-of-plane direction OP with the first end part 232 of the first rigid body 230. Correspondingly, the second end part 234 of the first rigid body 230 is coupled to a first section 224 of the second rotating mass part 212 through one or more springs 242-1, which are correspondingly configured to couple the first section 224 of the second rotating mass 212 part to move in the out-of-plane direction OP with the second end part 234 of the first rigid body 230. In other words, the springs 240-1, 242-1 are rigid in the out-of-plane direction OP, which is typically inherently the case if the springs and the mass parts are made of the same structure layer. Accordingly, acceleration upon the device structure in the out-of-plane direction OP causes the rotating mass parts 210, 212 to rotate about their respective rotary axes 218, 220. The circular curves of the rotary motion of the first sections 222, 224 in the rotating mass parts 210, 212 have a component in the out-of-plane direction OP, and components of motions of both of the rotating mass parts 210, 212 and motion of the first rigid body 230 are coupled. Due to symmetry of the structures, the components are essentially equal, so the resulting movement of the first rigid body 230 is linear motion in the out-of-plane direction. The linear motion reciprocates according to the balanced reciprocating motions of the two rotating mass parts 210, 212 about their respective rotary axes 218, 220.
For symmetry, in the example of
More detailed description of the rotating mass parts is provided in
Based on
It can now be noted that in the disclosed configuration, the center of mass of each of the rotating mass parts 210, 212 is offset from the respective rotary axis. Furthermore, springs of the spring structures can flex torsionally even if they were made of a thicker structure layer, and therefore be rigid in the out-of-plane direction OP. Accordingly, linear acceleration in the out-of-plane direction OP upon the device structure can efficiently transform into rotary motions of the two reversely rotating mass parts 210, 212. It is possible to capacitively detect these rotary motions with a static electrode positioned above and/or below the rotating mass structure. However, now also the considerable area of the linearly moving linear mass structure, which is coupled to move with out-of-plane motion of the rotating mass parts can be used for capacitive detection. The additional area of the linear mass structure makes the configuration capable to produce significantly larger detection signals. Furthermore, even if the requirement of asymmetry is fulfilled, the center of mass of the device structure is ideally in the point of symmetry of the coupled linear and rotating mass structures, and is therefore at, or in the immediate vicinity of the rotary axes of the rotating mass parts. Furthermore, as each of the rotating mass parts couples to the support through two coaxial torsional springs, and both of the rotating mass parts couple through spring structures to opposite ends of each elongate rigid body of the linear mass structure, the structure is highly balanced and very strongly resists in-plane rotation (rotation of the structure about an axis in the out-of-plane direction OP). This means that linear accelerations upon the device structure in the in-plane directions IP1, IP2 have very little leverage to transform into in-plane rotational movement. The balanced configuration is thus very robust against the most frequently occurring, and thus very harmful parasitic in-plane accelerations.
Angular momentum of a rotating mass is proportional to the mass and radius of rotation of the mass. Advantageously the radius of rotation, here essentially defined by the distance from the centre of the mass of the inertial masses in motion to the respective rotation axis, needs to be large enough to facilitate applicable response to the experienced accelerations. The volume of each rotating mass part can be concentrated to its outer edge, leaving free space between the outer edge and the rotary axis. This requirement facilitates compact configuration for a multi-axis accelerometer device etched from one structure layer area. The term ‘multi-axis’ in this context means that the device is configured to generate a signal in response to accelerations in two or more directions.
The internal configuration of X- and Y-elements is not relevant to the invention, as such. However,
Micromechanical device structures are typically surrounded by gas, with which the moving parts interact when in motion. Closed air gap heights between a device layer and the patterned support (capping and base) may be different and the thin films make it necessary to carefully consider damping effects of such gaps for operation. In a device structure, which is enclosed between a base and a capping to sense accelerations in the out-of-plane direction OP, and either of both of the base and the capping is patterned, it is often difficult to provide gas damping symmetrically from both sides of the structure.
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Claims
1. A microelectromechanical device including
- a support;
- a device structure that is flexibly coupled to the support; wherein
- in an initial static state of the device, the device structure forms a reference plane for the device;
- the support provides one or more stator electrodes and the device structure provides corresponding rotor electrodes configured for capacitive detection of acceleration in an out-of-plane direction that is perpendicular to the reference plane,
- the device structure includes a rotating mass structure and a linear mass structure;
- the rotating mass structure is formed of two rotating mass parts, a first rotating mass part and a second rotating mass part;
- the two rotating mass parts extend side by side in the reference plane such that in each of the two rotating mass parts, a first section of a rotating mass part extends outwards from a respective rotary axis;
- in each of the two rotating mass parts, the first section has the largest part of the inertial mass of the rotating mass part;
- the first rotating mass part is coupled to the support through a first spring structure and the second rotating mass part is coupled to the support through a second spring structure;
- each of the first spring structure and the second spring structure includes two torsional springs;
- a first end of a first torsional spring of the first spring structure is coupled to the support, a second end of the first torsional spring of the first spring structure is coupled to a first rotating mass part, a first end of a second torsional spring of the first spring structure is coupled to the support, a second end of the second torsional spring of the first spring structure is coupled to the first rotating mass part and the first and second torsional springs of the first spring structure are configured to twist about a first torsion axis;
- a first end of a first torsional spring of the second spring structure is coupled to the support, a second end of the first torsional spring of the second spring structure is coupled to a second rotating mass part, a first end of a second torsional spring of the second spring structure is coupled to the support, a second end of the second torsional spring of the second spring structure is coupled to the second rotating mass part and the first and second torsional springs of the second spring structure are configured to twist about a second torsion axis;
- the first torsion axis of the first and second torsional springs of the first spring structure is aligned to a rotary axis of the first rotating mass part;
- the second torsion axis of the first and second torsional springs of the second spring structure is aligned to a rotary axis of the second rotating mass part;
- the linear mass structure includes a first elongate rigid body and a second elongate rigid body, each of which extends in a second in-plane direction that is perpendicular to a first in-plane direction in the reference plane;
- in each of the two elongate rigid bodies, a section in one end of the elongate rigid body forms a first end part and a section in the opposite end of the elongate rigid body forms a second end part;
- a first end part of the first elongate rigid body and a first end part of the second elongate rigid body are coupled through a third spring structure to the first section of the first rotating mass part and a second end part of the first elongate rigid body and a second end part of the second elongate rigid body are coupled through a fourth spring structure to the first section of the second rotating mass part such that rotary motions of the first and second rotating mass parts transform into linear motion of the two elongate rigid bodies in the out-of-plane direction.
2. The microelectromechanical device of claim 1, wherein each of the first rotating mass part and the second rotating mass part has a U-shaped form, formed of two elongate vertical mass parts that extend in the second in-plane direction, each connected at a first end by an elongate horizontal mass part that extends in the first in-plane direction.
3. The microelectromechanical device according to claim 2, wherein
- the second end of the first torsional spring of the first spring structure is coupled to a side point in a first vertical mass part of the first rotating mass part;
- the second end of the second torsional spring of the first spring structure is coupled to a side point in a second vertical mass part of the first rotating mass part;
- the second end of the first torsional spring of the second spring structure is coupled to a side point in a first vertical mass part of the second rotating mass part;
- the second end of the second torsional spring of the second spring structure is coupled to a side point in a second vertical mass part the second rotating mass part.
4. The microelectromechanical device according to claim 3, wherein each side point is in a second end of its respective vertical mass part.
5. The microelectromechanical device according to claim 3, wherein each side point is separated by a non-zero distance from the second end of its respective vertical mass part.
6. The microelectromechanical device according to claim 1, wherein the rotary axis of the first rotating mass part and the rotary axis of the second rotating mass part are separated by a non-zero distance in a direction that is perpendicular to the direction of the rotary axes.
7. The microelectromechanical device according to claim 1, wherein the rotary axis of the first rotating mass part and the rotary axis of the second rotating mass part are aligned to a same line.
8. The microelectromechanical device according to claim 1, wherein
- each of the third spring structure and the fourth spring structure includes two springs;
- a first spring of the third spring structure couples the first end part of the first elongate rigid body to a connection point in the first section of the first rotating mass part;
- a second spring of the third spring structure couples the first end part of the second elongate rigid body to a connection point in the first section of the first rotating mass part;
- a first spring of the fourth spring structure couples the second end part of the first elongate rigid body to a connection point in the first section of the second rotating mass part;
- a second spring of the fourth spring structure couples the second end part of the second elongate rigid body to a connection point in the first section of the second rotating mass part;
- the connection points in the first section of the first rotating mass part have the same distance from the rotary axis of the first rotating mass part such that the first section of the first rotating mass part is coupled to move in the out-of-plane direction with motions of the first elongate rigid body and the second elongate rigid body;
- the connection points in the first section of the second rotating mass part have the same distance from the rotary axis of the second rotating mass part such that the first section of the second rotating mass part is coupled to move in the out-of-plane direction equally with motions of the first elongate rigid body and the second elongate rigid body.
9. The microelectromechanical device according to claim 1, wherein a structure formed of a combination of the rotating mass structure and the linear mass structure is centrally symmetric.
10. The microelectromechanical device according to claim 9, wherein a point of inversion of the centrally symmetric structure coincides with the centre of gravity of the structure.
11. The microelectromechanical device according to claim 10, wherein first ends of the first and second torsional springs of the first spring structure, and first ends of the first and second torsional springs of the second spring structure attach to a common elongate attachment structure.
12. The microelectromechanical device according to claim 11, wherein the elongate attachment structure includes an attachment point fixed to the support and two laterally extending rigid support structures that are not fixed to the support.
13. The microelectromechanical device according to claim 1, wherein
- the first end part of the first elongate rigid body and the first end part of the second elongate rigid body are coupled by a third elongate rigid body that extends along the reference plane in the first in-plane direction;
- the second end part of the first elongate rigid body and the second end part of the second elongate rigid body are coupled by a fourth elongate rigid body that extends along the reference plane in the first in-plane direction;
- the rigid bodies form a rigid frame that moves linearly in the out-of-plane direction according to rotational out-of-plane motion of the rotating mass parts.
14. A microelectromechanical device including
- a support;
- a device structure that is flexibly coupled to the support; wherein
- in an initial static state of the device, the device structure forms a reference plane for the device;
- the support provides one or more stator electrodes and the device structure provides corresponding rotor electrodes configured for capacitive detection of acceleration in an out-of-plane direction that is perpendicular to the reference plane,
- the device structure includes a rotating mass structure and a linear mass structure;
- the rotating mass structure is formed of two rotating mass parts, a first rotating mass part and a second rotating mass part;
- the two rotating mass parts extend side by side in the reference plane such that in each of the two rotating mass parts, a first section of a rotating mass part extends outwards from a respective rotary axis;
- in each of the two rotating mass parts, the first section has the largest part of the inertial mass of the rotating mass part;
- the first rotating mass part is coupled to the support through a first spring structure and the second rotating mass part is coupled to the support through a second spring structure;
- each of the first spring structure and the second spring structure includes two torsional springs;
- a first end of a first torsional spring of the first spring structure is coupled to the support, a second end of the first torsional spring of the first spring structure is coupled to a first rotating mass part, a first end of a second torsional spring of the first spring structure is coupled to the support, a second end of the second torsional spring of the first spring structure is coupled to the first rotating mass part and the first and second torsional springs of the first spring structure are configured to twist about a first torsion axis;
- a first end of a first torsional spring of the second spring structure is coupled to the support, a second end of the first torsional spring of the second spring structure is coupled to a second rotating mass part, a first end of a second torsional spring of the second spring structure is coupled to the support, a second end of the second torsional spring of the second spring structure is coupled to the second rotating mass part, and the first and second torsional springs of the second spring structure are configured to twist about a second torsion axis;
- the first torsion axis of the torsional springs of the first spring structure is aligned to a rotary axis of the first rotating mass part;
- the second torsion axis of the torsional springs of the second spring structure is aligned to a rotary axis of the second rotating mass part;
- the linear mass structure includes an elongate rigid body extending in a second in-plane direction that is perpendicular to a first in-plane direction in the reference plane;
- in the elongate rigid body, a section in one end of the elongate rigid body forms a first end part and a section in the opposite end of the elongate rigid body forms a second end part;
- a first end part of the elongate rigid body is coupled through a third spring structure to the first section of the first rotating mass part and a second end part of the elongate rigid body is coupled through a fourth spring structure to the first section of the second rotating mass part such that rotary motions of the first and second rotating mass parts transform into linear motion of the elongate rigid body in the out-of-plane direction.
15. The microelectromechanical device of claim 14, wherein each of the first rotating mass part and the second rotating mass part has a U-shaped form, formed of two elongate vertical mass parts that extend in the second in-plane direction, each connected at a first end by an elongate horizontal mass part that extends in the first in-plane direction.
16. The microelectromechanical device according to claim 14, wherein the rotary axis of the first rotating mass part and the rotary axis of the second rotating mass part are separated by a non-zero distance in a direction that is perpendicular to the direction of the rotary axes.
17. The microelectromechanical device according to claim 14, wherein
- the second end of the first torsional spring of the first spring structure is coupled to a side point in a first vertical mass part of the first rotating mass part;
- the second end of the second torsional spring of the first spring structure is coupled to a side point in a second vertical mass part of the first rotating mass part;
- the second end of the first torsional spring of the second spring structure is coupled to a side point in a first vertical mass part of the second rotating mass part;
- the second end of the second torsional spring of the second spring structure is coupled to a side point in a second vertical mass part the second rotating mass part.
18. The microelectromechanical device according to claim 17, wherein each side point is separated by a non-zero distance from the second end of its respective vertical mass part.
19. The microelectromechanical device according to claim 14, wherein
- each of the third spring structure and the fourth spring structure includes two springs;
- a first spring of the third spring structure couples the first end part of the elongate rigid body to a connection point in the first section of the first rotating mass part;
- a second spring of the third spring structure couples the first end part of the elongate rigid body to a connection point in the first section of the first rotating mass part;
- a first spring of the fourth spring structure couples the second end part of the elongate rigid body to a connection point in the first section of the second rotating mass part;
- a second spring of the fourth spring structure couples the second end part of the elongate rigid body to a connection point in the first section of the second rotating mass part;
- the connection points in the first section of the first rotating mass part have the same distance from the rotary axis of the first rotating mass part such that the first section of the first rotating mass part is coupled to move in the out-of-plane direction with motions of the elongate rigid body;
- the connection points in the first section of the second rotating mass part have the same distance from the rotary axis of the second rotating mass part such that the first section of the second rotating mass part is coupled to move in the out-of-plane direction equally with motions of the elongate rigid body.
20. The microelectromechanical device according to claim 14, wherein a structure formed of a combination of the rotating mass structure and the linear mass structure is centrally symmetric, and
- wherein a point of inversion of the centrally symmetric structure coincides with the centre of gravity of the structure.
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- Finnish Search Report dated Mar. 6, 2019 corresponding to Finnish Patent Application No. 20185646.
Type: Grant
Filed: Jul 16, 2019
Date of Patent: Aug 31, 2021
Patent Publication Number: 20200018777
Assignee: MURATA MANUFACTURING CO., LTD. (Nagaokakyo)
Inventors: Matti Liukku (Helsinki), Ville-Pekka Rytkönen (Klaukkala)
Primary Examiner: Walter L Lindsay, Jr.
Assistant Examiner: Philipmarcus T Fadul
Application Number: 16/512,946
International Classification: G01P 15/125 (20060101); G01P 15/08 (20060101);